Zapped by door knobs?

What's so special about the earth?
The reason you get a shock to earth is that it is large enough that the small amount of charge flowing from you doesn't change it's potential (voltage) so the charge can keep flowing.
Simimairly with a door knob, it's a relatively large isolated metal object so can contain a large amount of charge - so when you touch it there is a large potential difference which doesn't decrease, allowing all the charge to keep flowing from you.

It's a bit like touching a bath of hot water rather than a tiny spark. The spark is hotter but soon cools, the water can keep putting heat into you for longer.

 

You are the one that's charged. The doorknob is a relatively large metal object that doesn't mind having some extra electrons (it would rather have the electrons than your body). This causes any extra charge that you have accumulated to transfer to the doorknob, causing a shock.

This could happen with any metal object.

 

wow. I just had the same question today.
This also has to do with breakdown of air. It is 3MVolts/meter. In dry air this is lesser.
At 1mm, 3000V is good enough to cause a breakdown(spark) between you and a large enough conductor (metal door knob).
You can roughly calculate the voltage on your body by estimating the distance from the door knob, at which the spark occurs.
The door knob is like earth (a big reservoir of charges).